Libvisual is an abstraction library that comes between applications
and audio visualisation plugins.

Often when it comes to audio visualisation plugins or programs that
create visuals, they depend on a player or something else; basically,
there is no general framework that enabled application developers
to easily access cool audio visualisation plugins. Libvisual wants
to change this by providing an interface towards plugins and
applications; through this easy to use interface applications can
easily access plugins and, since the drawing is done by the
application, it also enables the developer to draw the visual
anywhere he wants.

The framework also allows you to morph to different plugins and
mix two at once; all kinds of neat tricks are possible using this
method.